Phase Behavior and Ion Transport in Lithium-Niobium-Tantalum Oxide Alloys

Lithium niobate-tantalate mixtures have garnered considerable interest for their ability to merge the desirable properties of both end members, enabling diverse high-value applications, such as high-performance faradaic capacitors, non-linear optics, and protective coatings in rechargeable batteries. While numerous studies on the application of $\mathrm{LiNb_xTa_{1-x}O_3}$ exist, the phase behavior and properties of $\mathrm{Li_3Nb_xTa_{1-x}O_4}$ remain largely unexplored. In this work, we employ a multiscale approach that encompasses first-principles phonon calculations, cluster expansion, and Monte Carlo simulations to derive the temperature-composition phase diagram for $\mathrm{Li_3Nb_xTa_{1-x}O_4}$. Our findings reveal the critical role of vibrational entropy in accurately predicting phase stability, which promotes the solubility of Nb in $\mathrm{Li_3TaO_4}$ while suppressing the miscibility of Ta in $\mathrm{Li_3NbO_4}$. Additionally, we demonstrate that Nb/Ta mixing offers a promising avenue for tailoring the Li-ion conductivities of $\mathrm{Li_3Nb_xTa_{1-x}O_4}$. On the technical side, we demonstrated the importance of including vibrational entropy effects explicitly in Monte Carlo simulations dealing with multicomponent systems, beyond simple binary mixtures. On the application side, this study provides fundamental insights into the phase behavior and Li-ion transport properties of $\mathrm{Li_3Nb_xTa_{1-x}O_4}$, paving the way for its potential applications in energy storage and other fields.